Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder subluxation, etc. The description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227. However, the present invention may find applicability in any implantable medical device system.
As shown in FIG. 1A, an SCS system typically includes an Implantable Pulse Generator (IPG) 10, which includes a biocompatible device case 12 formed of a conductive material such as titanium for example. The case 12 typically holds the circuitry and battery 14 necessary for the IPG to function, as described in detail below. The IPG 10 is coupled to distal electrodes 16 designed to contact a patient's tissue. The distal electrodes 16 are coupled to the IPG 10 via one or more electrode leads (two such leads 18 and 20 are shown), such that the electrodes 16 form an electrode array 22. The electrodes 16 are carried on a flexible body 24, which also houses the individual signal wires 26 coupled to each electrode. In the illustrated embodiment, there are eight electrodes on each of leads 18 and 20, although the number of leads and electrodes is application specific and therefore can vary. The leads 18 and 20 contain proximal electrode contacts 29 that couple to the IPG 10 using lead connectors 28 fixed in a non-conductive header material 30 such as an epoxy.
As shown in the cross-section of FIG. 1B, an IPG 10 typically includes a printed circuit board (PCB) 32 to which various electronic components 34 are mounted, some of which are discussed below. A telemetry (antenna) coil 36 is used to transmit/receive data to/from an external device (not shown) through the patient's tissue. Such wireless communications can occur using different hardware and protocols, such as Frequency Shift Keying, Bluetooth, Bluetooth Low Energy, WiFi, Zigbee, etc. See, e.g., U.S. Patent Application Publication No. 2015/0073498.
IPG 10 in the depicted example contains a non-rechargeable primary battery 14p (where “p” denotes “primary”). Unlike a rechargeable battery, the electrochemical reaction in a primary battery 14p is not reversible by passing a charging current therethrough. Instead, a primary battery 14p will eventually expend the materials in one or both of its electrodes, and thus has a limited life span. Once the battery 14p is exhausted, it will be necessary to explant IPG 10 from the patient so that the battery 14p can be replaced and the IPG 10 re-implanted, or (more likely) so that a new IPG 10 with a fresh battery 14p can be implanted. Primary batteries 14p can be formed using different chemistries, but Lithium CFx batteries or Lithium/CFx-SVO (Silver Vanadium Oxide) hybrid batteries are popular for use in implantable medical devices, and produce battery voltages, Vbat, between 1.2 and 3.2 Volts in one example. As described further with respect to FIG. 3B below, Vbat will decrease over time as the primary battery 14p is used over the life of the IPG 10.
FIG. 2 shows an architecture 5 for IPG 10, which is described in U.S. Patent Application Publication 2013/0331910, and which is incorporated herein by reference. Shown with particular emphasis are the various power supplies in the IPG 10, which are shown with thicker lines. Only a few other non-power supply signals are shown in FIG. 2 to the extent they are discussed below, and such signals are shown with thinner lines. One skilled in the art will appreciate that the IPG 10 contains many such “regular” signal lines, and may contain other power supplies, which are not shown for convenience.
Primary battery 14p provides the main power supply voltage, Vbat, from which all other power supply voltages in the IPG 10 are derived. Some circuits, such as a DC-DC converter 62 and tank circuitry 68, described further below, receive power directly from Vbat. Other power supplies voltages needed in IPG 10 may be higher than Vbat, and so are generated using a boost converter 70, which produces a higher power supply voltage, Vup, from Vbat. Vup may be 3.2 V in one example. Boost converter 70 can comprise a capacitor-based change pump, an inductor-based step-up converter, or a combination of these. See, e.g., U.S. Pat. No. 7,872,884.
As shown, architecture 5 contains analog circuitry 52, digital circuitry 54, and memory 60, each of which are powered by their own power supply voltages Vd, Va, and Vf, each of which is generated from Vup via regulators 44, 46, and 48. Regulators 44, 46, and 48 can comprise low drop out linear regulators that use little power and that create less noise than switching regulators. Even if the power supply voltages Vd, Va, and Vf are of the same magnitude (e.g., 2.8 V) in the IPG 10, it is useful to isolate them via the regulators to prevent noise on one power supply from affecting the other. For example, digital circuitry 54 may create noise on Vd as it switches, which noise could potentially affect Va and hence performance of analog circuitry 52 if not isolated. Memory 60 (e.g., a Flash EPROM) preferably has its own regulator 48 because it may consume a large amount of current (e.g., when being programmed or erased), which Vf and regulator 48 must supply. Additionally, regulator 48 can be disabled from time to time to save power.
Analog circuitry 52 powered by Va includes a number of circuits, including thermistors for measuring temperature (T), band gap generators for producing temperature-insensitive reference voltages (Vref), oscillators and clock generation circuitry (CLK), telemetry circuitry 72 including modulation and demodulation circuitry, analog measurement circuitry 74, and the like.
Digital circuitry 54 comprises the digital circuits in the IPG 10 that are powered by power supply voltage Vd, and include a microcontroller 58 and various timing circuits 56. Digital circuitry 54 can be integrated, at least in part, on a single mixed-mode Application Specific Integrated Circuit (ASIC) with at least some of the analog circuitry 52, as shown for example in U.S. Patent Application Publications 2008/0319497 and 2012/0095529. Digital circuitry 52 can also be discrete from the analog circuitry 54. For example, the ASIC of the '497 and '529 Publications can used in conjunction with Texas Instruments microcontroller 58 part number MSP 430 for example, which is described in data sheets stored on Texas Instruments' website, which data sheets are incorporated herein by reference. Microcontroller 58 may also comprise a microprocessor integrated circuit, a collection of integrated circuits, a collection of non-integrated circuits, or a collection of both integrated and non-integrated circuits—essentially any hardware capable of operating the IPG 10 in the manners disclosed herein.
Memory 60, powered by power supply voltage Vf, can hold the operating software for the IPG 10 (e.g., for the microcontroller 58), and can also act as a free space to store data, such as logged data to be wirelessly telemetered to an external device for analysis and/or to provide feedback to the patient, clinician, or IPG manufacturer. Memory 60 can also store data transmitted from an external device, such as the therapy settings for a given patient, such as which electrodes 16 are active to provide current pulses, and the magnitude, polarity, frequency, and, width of those pulses. Such therapy settings are sent from the microcontroller 58 to one or more Digital-to-Analog Converters (DAC(s)) 64, which generate the specified current pulses accordingly (Iout). See, e.g., U.S. Patent Application Publication 2007/0100399. Only one electrode 16 is shown in FIG. 2 for simplicity, but one skilled on the art will understand that therapy settings would involve more than one electrode. For example, another electrode 16 (which could include the conductive case 12; FIG. 1A) would be used to provide a return path for Iout to prevent the build-up of charge in the patient's tissue. Additionally, more than one electrode 16 may be used to source current to the patient's tissue, and more than one electrode 16 may be used to sink current from the patient's tissue. See, e.g., U.S. Patent Application Publications 2013/0184794 and 2013/0006315.
The power (e.g., magnitude) of the current pulses at the electrodes 16 can differ from time to time for a given patient, or from patient to patient. Accordingly, the power supply voltage for the DAC(s) 64, called the compliance voltage (V+), is not fixed, but is instead set at an optimal level via a feedback loop. V+ monitor and adjust circuit 66 monitors a voltage in line with the active electrode(s) 16, which it uses to control the DC-DC converter 62 to generate a power supply voltage V+ of an appropriate magnitude for the DAC(s) 64. See, e.g., U.S. Pat. No. 7,444,181, U.S. Patent Application Publications 2010/0211132, 2013/0289665, and 2013/0310897. Such adjustment of V+ is desired because if V+ is too low, DAC(s) 64 will become “loaded” and unable to provide the specified current, Tout. If V+ is too high, DAC(s) 64 will be able to provide the desired current, but power will be wasted, with some portion of the compliance voltage V+ being dropped across the DAC(s) 64 without any useful effect. See, e.g., U.S. Pat. Nos. 7,539,538 and 7,872,884. As noted earlier, DC-DC converter 62 produces V+ directly from Vbat, and the converter 62 (like boost converter 70) can comprise a capacitor-based change pump, an inductor-based step-up converter, or combination of these. V+ may be set by the converter 62 in one example between 3 to 18 Volts, again depending on power of the current the DAC(s) 64 must provide.
Tank circuitry 68 is coupled to the telemetry coil 36, and like DC-DC converter 62 is directly powered by Vbat. Tank circuitry 68 can comprise a tuning capacitor (not shown) which operates in conjunction with the inductance of the coil 36 to set the tank's resonant frequency as necessary for communications with an external device. Tank circuitry 68 also includes transistor switches controlled by modulation circuitry within telemetry circuitry 72, which modulation circuitry converts digital data from the microcontroller 58 to be transmitted to signals that switch the transistor switches at or near the resonant frequency. When receiving data, the tank circuitry 68 is coupled to demodulation circuitry within telemetry circuitry 72 for extracting the digital data from the received signal for interpretation by the microcontroller 58. See, e.g., U.S. Patent Application Publication U.S. 2009/0069869 and U.S. Pat. No. 8,081,925.
An issue of concern to IPG manufacturers is IPG quality and reliability, and in this regard manufacturers often employ testing to ensure that IPGs are not defective. For example, prior to insertion of the IPG's circuitry within its case 12, when the circuitry is still accessible and before the battery 14p is connected to it, manufacturers often run various tests to see whether the IPG is drawing too much current. This could occur for example if a defect in the IPG circuitry is causing current leakage from a power supply voltage (or other voltages) to ground. If such current leakage is present, the IPG 10 may simply not work, or the primary battery 14p, once connected, may deplete more quickly than it should. Faster-than-normal battery depletion is particularly concerning for primary-battery IPGs, because as noted earlier, the IPG must be explanted when the battery is expended, with substantial inconvenience to the patient.
An example of IPG testing at this stage is shown in FIG. 3A. A tester 75 is coupled to the Vbat node in the IPG circuitry (i.e., the node to which the positive terminal of the battery 14p will ultimately be connected). The tester 75 biases this node to a proper voltage to power the IPG 10 (e.g., Vbat=3.0V), and measures the current draw through that node, Ibat, which comprises the total current drawn by the IPG 10. Although not shown, tester 75 would also likely couple to other nodes such as a connector in the IPG circuitry to provide the signaling necessary to operate the IPG 10 normally, thus allowing Ibat to be determined under realistic use conditions. If Ibat as determined by the tester 75 is unusually high (e.g., above a threshold Ibat′), the IPG 10 under test may be deemed faulty, and thus not further manufactured or shipped. One skilled in the art will recognize that other tests of the IPG 10 can be run at this stage beyond determination of Ibat.
However, testing currents is difficult to accomplish once the IPG circuitry has been connected to its battery 14p, sealed inside its case 12, and is no longer accessible. Nonetheless, the inventors consider it desirable to determine such currents at this stage, because manufacturing steps associated with encasing the IPG circuitry and connecting additional structures (such the battery 14p) can give rise to additional defects that are desirable to discover.
A method employed by IPG manufacturers to infer IPG current draw at this stage, at least for IPGs having wirelessly-rechargeable batteries 14r (where “r” denotes “rechargeable”) (see FIGS. 11A-11C), is to: charge the battery 14r; wirelessly transmit its voltage (Vbat1) to an external device; operate the IPG 10 normally for some period of time (e.g., a one week “burn-in”); wirelessly transmit its voltage thereafter (Vbat2); and then determine the difference in the battery voltage before and after (ΔVbat=Vbat1−Vbat2). The resulting ΔVbat can then be compared to a threshold (Vbat(th)), and if ΔVbat>ΔVbat(th), the manufacturer can conclude that the IPG 10 is drawing too much current from the battery 14r, and that the IPG 10 is not acceptable at least from this perspective.
This approach however is problematic when applied to the testing of a primary-battery IPG for a number of reasons. First, and as shown in FIG. 3B, the discharge curve of a primary battery (Vbat as a function of time, assuming a constant current draw) is non-linear. It comprises a drop-off portion 76 where Vbat falls relatively quickly early in its use, and later enters a relatively flat portion 78 where Vbat falls slowly, at least until the End Of Life (EOL) of the battery is reached (i.e., at voltage Vbat_EOL, at which point the battery 14p can no longer operate the IPG's circuitry). Experience teaches that primary batteries 14p when purchased new are not consistent in where they seem to be currently operating on this discharge curve. Some new batteries seem to be at the beginning of their drop-off portion 76, and thus fall quickly upon use, while others seem to have surpassed the drop-off portion 76 to at least some degree, and are thus closer to, or in, the flat portion 78. Thus, for the same burn-in time, different values for ΔVbat could result, even if the primary-battery IPGs being tested in fact are drawing the same amount of current, Ibat. In other words, ΔVbat is not a good predictor of current draw for a primary battery 14p. 
Second, Vbat for a primary battery 14p will fall slowly compared to a rechargeable battery 14r, even if the primary battery 14p is still within its drop-off portion 76. As a result, a meaningful burn-in time for primary-battery IPGs would need to be substantially longer than a week to see significant differences in ΔVbat, such as months, which is impractical for the manufacturer. Moreover, the months of burn-in permanently deplete the primary battery 14p to some degree, which amounts to shortening the useful life of the IPG 10 by an equivalent amount. (Note that this is not a concern for a rechargeable-battery IPG 10, because after a relatively-short burn-in period, an acceptable IPG 10 exhibiting a suitably small ΔVbat can have its battery 14r recharged before the IPG 10 is shipped).
The inventors have conceived of ways to measure current draws after primary-battery IPG 10 manufacturing is complete, and one way is shown in FIG. 3C (which may be inventive and is not admitted as prior art). In this example, current measurement circuitry 55 includes a measurement resistor Rm placed in line with Ibat between the battery 14p and the remainder of the IPG circuitry. Rm would preferably be small (e.g., 1 ohm) so as not to waste battery power (e.g., Ibat2*Rm), and may comprise an already-existing component in the path, such as a fuse. As Ibat passes through Rm, a voltage Vm (Ibat*Rm) builds up across Rm, which can be measured with a differential amplifier 73. This measurement voltage Vm can be digitized at an Analog-to-Digital (A/D) converter 74, which comprises part of measurement circuitry 74 (FIG. 2), and which is usually present in an IPG 10 and used for other reasons. Thereafter, microcontroller 58 can wirelessly transmit Vm (or Ibat, by dividing Vm by the known value of Rm) to an external device using telemetry circuitry 72 for the manufacturer's review.
As such, a manufacturer can use the current measurement circuitry 55 of FIG. 3C to assess Ibat even after IPG manufacture is complete, and thus can determine whether Ibat is high enough to suggest a defect and thus that the IPG should not be shipped. Advantageously, this measurement occurs without the need of a burn-in period of long duration that would at least partially deplete the primary battery 14p. 
However, while the current measurement circuitry 55 of FIG. 3C is certainly plausible, it is not necessarily realistic to implement in all IPG designs. This is in part due to the varying nature of Ibat during normal operation of the IPG 10. As shown in FIG. 3D, Ibat is not generally constant, but contains high-current draw spikes (Itelem). Indeed, as the logarithmic y-axis scale in FIG. 3D makes clear, these current spikes can be 1000 times larger than the baseline current (Ib) between them.
These current spikes result from the communication scheme used between the IPG 10 and an external device, which is discussed in U.S. Pat. Nos. 7,725,194 and 8,131,377, which are briefly explained. While the IPG 10 is designed to wirelessly communicate with an external device, it is not practical to constantly provide power to the IPG's telemetry circuitry 72 to allow the IPG 10 to determine when an external device is attempting to communicating with it. Receiver circuitry within the IPG's telemetry circuitry 72 draws too much power to permit this. Instead, the IPG 10 powers its receiver circuitry only during listening windows of a small duration (e.g., Td=20 msec), which issue periodically (e.g., Tp=0.5 s). It is incumbent on the external device wishing to communicate with the IPG 10 to continuously broadcast a wake-up signal for a duration of at least Tp, and preferably longer, to ensure that the IPG 10 can sense such wake-up signal during at least one of its listening windows. If the IPG 10 so senses the wake-up signal, it can then power its receiver circuitry (and possibly also its transmission circuitry if the IPG 10 needs to transmit data) for as long as necessary to communicate with the external device during a communication session, after which it can revert to periodically issuing listening windows once again.
Thus, Ibat in FIG. 3D is high during these listening window periods (Td) as the telemetry circuitry 72 draws current Itelem from power supply voltage Va. By comparison, the base line current Ib drawn between these windows is relatively small, when the IPG is otherwise drawing current from various analog 52, digital 54, and memory 60 circuitry during its normal operation, such as providing patient therapy.
While current measurement circuitry 55 of FIG. 3C theoretically allows Ibat to be determined, Ibat is not so easily determined in the IPG 10 as a practical matter. First, the differential amplifier 73 must have a large dynamic range able to accurately resolve small voltages over the orders of magnitude by which Ibat varies. For example, for typical Ibat currents (from Ib to Itelem), and assuming a measuring resistor of Rm=1 ohm, Vm will range from a few microVolts to a few milliVolts, which voltages are difficult to measure.
Second, the A/D 74 may not be well suited to meaningfully interpret Ibat via Vm. Although an A/D 74 is typically present in an IPG 10 to measure various voltages during IPG operation, A/D 74 may not be configured to sample measured voltages at a rate necessary to resolve Ibat. In this regard, Ibat can have current variations of even shorter durations that the listening windows, for example, as small as microseconds in duration. If one assumes that A/D 74 must capture current variations in Ibat as small as one microsecond, it must sample Ibat at least every 0.5 microseconds (per well-known sampling rules). This would amount to two million samples per second—a large amount of data that the IPG 10 may not be able to store. This problem is exacerbated by the need to sample Ibat for long enough to pick up at least a few of the current spikes, and thus Ibat may need to be sampled for longer than one second.
High sampling rates would also tax the A/D 74, which typically doesn't need to sample data at this rate during normal use in the IPG 10, and which may therefore be unable to do so. Even if A/D 74 can be configured to sample Ibat at an acceptable rate for test purposes, A/D 74, by virtue of its faster operation, could start to draw significant current, as could other circuitry in the IPG 10 attempting to handle the large amounts of data A/D 74 is providing. Such atypical use of the IPG 10 during this measurement would therefore increase Ibat, and would thus skew its measurement, as Ibat would not be reflective of normal IPG operation.
Finally, while the current measurement circuitry 55 of FIG. 3C is capable of measuring Ibat being drawn from power supply voltage Vbat, the inventors consider it desirable to determine the current draws from other power supply voltages in the IPG 10. This would be beneficial, as it would allow the manufacturer to understand which circuits in the IPG might be drawing excessive currents. For example, if the current being drawn from power supply voltage Vup is unusually high, it may suggest a problem in the regulators 44, 46, or 48 that draw current from this power supply voltage. Such excessive currents would ultimately contribute to Ibat, and thus may be noticeable in its measurement, but Ibat would be unable to reveal the source of the current leakage. Additionally, current leakage from a particular power supply voltage may be relatively small and thus not resolvable in Ibat, even if such leakage is still significant to the circuitry powered by that power supply voltage, which circuitry may be at least at risk of failure. Thus, the inventors recognize that the current measurement circuitry 55 of FIG. 3C could be included in line with all of the power supply voltage to determine their current draws, but again, a more practical current-measuring solution is desired.